1. Field of the Invention
This invention relates to a threadedly fixing apparatus which is used to threadedly mount a connecting coupling on one end of a relatively large diameter pipe such as oil well pipes for boring petroleum, pipes used for various civil engineering works and the like, or which is used to threadedly mount a protector on the coupling.
2. Description of the Prior Art
Such a pipe as described above is machined in factories so that external threads are cut on opposite ends thereof, after which a protector for protecting the external threads is threadedly mounted on one end thereof and one end of a connecting coupling is threadedly mounted on the other end. A protector for protecting internal threads is threadedly mounted on the other end of the coupling. The thus processed pipes are supplied to the site for use. Incidentally, the aforesaid external threads cut on the pipe and the internal threads of the coupling to be threadedly fitted therein are all buttress tapered threads. One reason is that in the state where one pipe is connected to the other by use of a coupling, it is possible to transmit a great turning force from one pipe to the other. Another reason is that in the above-described connected state, good sealing properties between inside and outside of the pipe are maintained. Since the buttress tapered threads are used as the external threads of the pipe and as the internal threads of the coupling as described above, similar buttress tapered threads are also used for the protector for protecting these threads. However, the above-described buttress tapered threads pose a problem. That is, threadedly fitting work between the buttress tapered external threads and buttress tapered internal threads is very difficult. This will be explained. Threadedly fitting between the tapered external threads and tapered internal threads is carried out in the following process. First, an external thread 57m is positioned inside an internal thread 54m as shown in FIG. 20. Next, both the threads are threadedly fitted by relative rotation of both the threads. In this case, all of a number of threads of one thread have to be simultaneously fitted into a number of thread grooves of the other. To this end, the relative positions between the aforesaid number of threads and the number of thread grooves have to be placed in coincidence with each other accurately. It is very difficult to accurately position them so as to threadedly fitting both the threads, as described above. If the aforesaid relative position is slightly deviated as shown in FIG. 20, the number of threads 54n of one thread and the number of threads 57n of the other become bit at their respective crests when both the threads are relatively rotated. If such biting should occur, the number of threads are broken or damaged when it is removed. As a result, there gives rise to problems in that the ability for transmission of the turning force is declined and the sealing properties are deteriorated, as previously mentioned.